Tuesday, June 24, 2008

JAYPHAT Impedance Matcher--Pt. 5

...OF MICE and MEN (or DRATS!)--

Part five sucks! Son, John ask if we could travel over to College Station and take in a movie with his sister, Erica, so I finished slapping the Jayphat together and just had time to plug it into my amp for a test run. Zilch, nothing, nada--no signal what-so-ever was being emitted. So, I left for the movies and have temporarily re-named my box the Jaysplat. I am glad that we took in Get Smart for the matinee. It was quite hilarious. Hollywood is really trying to get us babyboomers to recall our youth, from our comic book heroes to cheesy TV sitcoms. I THINK I'll have time to troubleshoot my missteps in my wiring (film at 11). Anyway--

3 comments:

HTownFess said...

Rick, take another look at the schematic and think about it conceptually. You've got the signal coming in with 4.4 Meg of grid loading, going into pin 3 on the JFET. Make sure that resistance is downstream of the input stage, same as on a grid leak tube stage. Then you've got the cathode equivalent connected to pin 2, going to ground.

What hooks to pin 3 is the equivalent of the plate load resistance and power supply dropping resistance on a tube stage. The 100 mfd cap is really a filter cap, the 10 ohm resistor just isolates that from the battery. Same as on a tube stage, the series resistors should hook to pin 1 closer than the 5 mfd coupling cap.

Gotta get the terminal connections correct on yer pot. Believe it's the same as on a mic volume pot since this pot does a similar job of providing resistance to ground when turned up.

The original Jayphat has got the - (negative) battery connection going to the sleeve lug on the output jack, same spot as the ground terminal on the pot goes to. Dunno if that's crucial, but do it that way.

Lack of any sound suggests that either the signal is going to ground prematurely, or power is not getting to the JFET. Trace both possibilities. I think it's pretty hard to fry the JFET with just 18v available.

Check the dumb stuff like tip/sleeve on the jacks too, plus their isolation, any dang thang. Make sure the components on the perf board can't accidentally ground to the enclosure. Battery voltage, etc. Patch cords can fool you--they love to give out in this situation, I've had at least three do that.

Your experience suggests that @ $0.38 apiece, buying more than one JFET might not be a bad idea when gathering supplies. I wonder if somebody at Mouser is currently scratching their heads at a small spike in MPF102 sales--

HTownFess said...

Correction in the first paragraph: "downstream of the input capacitor"

Ricky Bush said...

Thanks, Professor! Yeah, I've done something dumb somewhere. I fooled around a bit this morning with it and it gave me a headache. Gonna tackle it again this afternoon.

Oh, and I did order an extra JFET--just in case (this case). I think I ordered two of everthing--just in case I needed to start over from scratch. Which I may just do if I get irritated this afternoon. I did notice this morning before reading your comments that I wired up the pins to the JFET wrong. Re-wire improved nothing, though